It’s not clear exactly what happened, but union officials say the engineer lost focus moments before his commuter train jumped the tracks as it approached a sharp curve at a high speed. New York Gov. Andrew Cuomo and other officials have raised the possibility of bringing charges against the engineer, William Rockefeller.

But making any kind of case presents hurdles for prosecutors. That’s because falling asleep, or daydreaming, behind the wheel (if that’s what happened) and causing a crash doesn’t automatically establish criminal liability.

Often, officials need some proof of what was going on inside the head of the defendant just before the crash, says criminal law expert Arnold H. Loewy of Texas Tech University School of Law. And gathering enough proof can be a challenge. “It’s tricky because you often don’t know what his state of mind is,” he told Law Blog.

An attorney for Mr. Rockefeller didn’t respond to a request for comment on Wednesday.

A person can be found guilty of criminally negligent homicide if it’s determined that he did something dangerous that puts people’s lives in serious jeopardy but isn’t aware of the obvious risks. A reasonable person would know not to do it. In New York, it’s a Class E felony that carries a maximum punishment of 4 years in prison.

A higher degree of negligence is required for a second-degree manslaughter conviction. If a person is aware that his actions could very well kill someone but without justification takes the risk anyway, that’s recklessness. Someone charged with that offense could face 15 years.